CALGARY — The chairman of TransAlta Corp. says a lawsuit launched by an American activist investor on Tuesday is a "frivolous tactic" and the power utility intends to defend itself vigorously.

Gordon Giffin says the move by New York-based Mangrove Partners to prevent the Calgary-based company from concluding a $750-million partnership deal with Brookfield Renewable Partners won't succeed.

Mangrove is trying to derail the deal because it says it was rushed, failed to properly consider other options and puts the interests of the board ahead of the interests of the shareholders.

Last week, the investor withdrew an application to the Alberta Securities Commission for a hearing to demand a delay in the meeting at which the company plans to ask shareholders to elect Brookfield nominees to its board of directors, a condition of closing the agreement. It also wanted TransAlta to be forced to hold a vote on the deal.

Giffin says the majority of TransAlta's shareholders have already voted "overwhelmingly" in favour of its nominated board.

TransAlta CEO Dawn Farrell has defended the Brookfield deal as very good for shareholders previously and insisted the company thoroughly investigated other options before signing.

Mangrove and an entity controlled by Bluescape Energy Partners had agreed to join forces to use their combined 10.1 per cent ownership of TransAlta to force the examination of other financial options but they said in a regulatory filing last Friday that they had ended their partnership. Mangrove says it owns 7.1 per cent of TransAlta.

TransAlta has said it plans to spend $350 million of the Brookfield investment to speed its coal-to-gas power generation transition strategy in Alberta and will use up to $250 million to buy back shares over three years.